1. Field of the Invention
The present invention relates to an illumination system for a projection display, and more particularly, relates to a hybrid illumination system and a light integrating device for the hybrid illumination system.
2. Descriptions of the Related Art
The rapid development of science and technology has led to the constant improvement of information display technology and an increasingly widespread application of projection systems. Accordingly, a high demand is placed on the imaging quality of the projection systems by users. One of the important factors of evaluating the imaging quality is the brightness of an image. The brightness of the image usually depends on the luminance of the light source, light collecting efficiency, light uniformizing efficiency, light relaying efficiency and similar factors. When the luminous efficiency is restricted by the étendue of a projection system, one of the solutions for providing better luminous power is to provide a brighter and more efficient light source. However, such a light source is often very expensive and has a short life cycle. Furthermore, it usually has a bulky volume, which further increases the overall dimensions of the system.
In a high-luminance projection system, a xenon bulb or a number of ultra high pressure mercury lamps (UHPs) are often used as the light source. However, the xenon bulb has a short life cycle and a higher cost. On the other hand, when a number of UHPs are used as the light source, it would impose difficulty on the designs and applications of a system to minimize the lost of luminous efficiency due to the increasing étendue of the system. Moreover, in terms of spectrum output, UHPs have complete spectrum distributions and high intensity in the blue and green light region, while the output in red light has an incomplete spectrum and insufficient intensity.
To reduce costs, UHPs in combination with an LED or a laser are commonly used to form a hybrid illumination system to reinforce the red spectrum of the UHPs. Compared to LEDs, lasers have narrow bandwidth and with a very small étendue. Hence, if UHPs are employed to output the blue and green lights, while the red laser is employed to reinforce the insufficient red lights, luminance in the red light region can be even higher than that of the xenon bulb and a higher overall illumination efficiency can be obtained. Therefore, a hybrid light source consisting of UHPs and a red laser can not only overcome the restriction of luminance provided by a number of UHPs, but also provide a wider color gamut due to the addition of the red laser.
FIG. 1 illustrates a conventional hybrid illumination system 1, which comprises an UHP 11, two laser sources 12, a first lens array 13, a second lens array 14, a panel 15 and two reflecting prisms 16. The light source of the hybrid illumination system 1 is a mix of light emitted from the UHP 11 and the laser source 12. More specifically, the first and the second lens arrays 13, 14 are disposed between the UHP 11 and the panel 15. Light emitted from the UHP 11 travel through several lenslets of the first lens array 13 and then focus on corresponding lenslets of the second lens array 14. Then, light transmitting through individual lenslets of the second lens array 14 are all deflected and projected to the panel 15 with the light overlapping each other on the panel 15. On the other hand, light emitted from the laser source 12 is condensed and reflected by the reflecting prism 16 to the lenslets of the first lens array 13. Likewise, these lights are also projected by the corresponding lenslets of the second lens array 14 onto the panel 15 to overlap with the light from the UHP 11. Although the above structure may mix light, this illumination system has a prominent shortcoming; specifically, the UHP 11 and the laser source 12 have different étendues, i.e. a portion of the light will be discarded rather than become effective light, so the overall illumination efficiency of the system is decreased. Furthermore, the lasers disposed on both sides extend not only the étendue, but also the volume of the system.
FIG. 2 depicts another conventional hybrid illumination system 2, which comprises an UHP 21, a laser source 22 and a dichroic beam splitter 23. The red light supplied by the laser source 22 is reflected by the dichroic beam splitter 23 before being incorporated into the illuminating path of the UHP 21. This method of mixing light sources may avoid the different étendues between the different light sources as was seen in the hybrid illumination system 1. However, to use such a hybrid illumination system, the use of an additional dichroic beam splitter 23 or other relay optics must be considered in the early steps of designing the light path of the system, making the completed system design less flexible during modification. Consequently, when being applied to another illumination system or adding other different light sources as desired, the system structure often has to be redesigned.
In summary, the conventional hybrid illumination systems used in projection displays either fail to provide high illumination efficiency due to the additional étendues by the additional light sources (laser source) or lack flexibility in modifying the étendues of the conventional light source. In view of this, it is important to provide a hybrid illumination system that adds no additional étendues and provides the flexibility for combining other light sources.